Carbon makes super-tough paper

Researchers have used a recently discovered form of carbon to make sheets of super-stiff and super-strong paper that is only
a few millionths of a metre thick. This paper could lead to the development of tough new materials or be used to store energy
for fuel cell applications.

Graphene, discovered in 2004, is tougher than diamond. It is composed of atom-thick sheets of carbon atoms arranged in a two-dimensional
honeycomb structure. But unless they are anchored to some other material, these sheets spontaneously scrunch up into little
balls and form a graphene 'soup'.

Rodney Ruoff, at Northwestern University in Evanston, Illinois, and his colleagues have now found a way of making free-standing
sheets of graphene 'paper'.

The researchers took graphite oxide, a layered material, split it into microscopic individual plates, and then used a flow
of water to stick these tiny plates together in a flat paper-like structure. Water acts as a glue between the layers of graphene,
because hydrogen bonds form between the water molecules and the graphene strips.

The result was a sheet of graphene oxide, consisting of overlapping particles a bit like the scales of a fish. These sheets
were only a few micrometres thick. "You can't make non-coagulating graphene but you can make non-coagulating graphene oxide,"
says Andre Geim, a graphene pioneer at the University of Manchester, UK. The graphene oxide will have similar chemical properties
to pure graphene, he says.

Ruoff tested his paper against bucky paper (based on C60), flexible graphite and vermiculite, and says that his paper outperforms the other carbon-based papers in terms of tensile
strength and stiffness. These papers are used commercially as lubricants and seals for gaskets.

Like normal paper, the sheets are stiff and strong in one direction — thanks to the hydrogen bonds holding the small pieces
of graphene together — but bendy. "In plane they're very stiff, but they're also very compliant," says Ruoff, "so they can
be folded."

Sticking point

The paper could be made even stronger and stiffer by tinkering with the glue between the layers, says Ruoff — that is, by
replacing the hydrogen bonds with other, stronger groups. "If you want to make the stiffest, strongest material ever you need
to choose components at the molecular level that are strong and tough," he says.

Geim agrees: "What immediately comes to mind is you can think about really good binders for the [graphene] planes and make
very tough materials."

Applications are still a long way off, but could be wide-ranging, says Ruoff. For example, the paper could form large membranes
in conducting materials; or the gap between the layers where the molecular 'glue' sits could be used to as a way to store
energy for fuel cells, he says.